1. Technical Field
The invention relates to vehicles and, more particularly, to structures for reducing aerodynamic drag on moving vehicles.
2. Discussion of Art
Aerodynamic drag is a long-standing problem relative to wheeled vehicle fuel efficiency. At typical road travel speeds, aerodynamic drag on a wheeled vehicle exceeds tire rolling resistance as a load on the vehicle engine. Thus, engineers have long recognized that it is desirable to reduce the aerodynamic drag of a wheeled vehicle.
Various approaches have been attempted toward reducing aerodynamic drag. Most commonly, wheeled vehicle bodies are streamlined to mitigate flow separation. Flow separation occurs at a place along a vehicle body where a boundary layer of air separates from the surface of the vehicle body due to the boundary layer moving slower than surrounding air. The place where flow separation occurs, will vary based on vehicle speed. Flow separation produces turbulent volumes of air adjacent the vehicle body, and these turbulent volumes effectively increase the cross-section of the vehicle body, thereby increasing drag. Streamlining delays flow separation to a place further along the vehicle body, thereby reducing the turbulent volumes of air and the resultant cross-section of the vehicle body. The effectiveness of streamlining will vary based on vehicle speed.
Efforts have been made to further delay flow separation by introducing forward-mounted spoilers. For example, Elder's U.S. Pat. No. 4,360,232 shows an aerodynamic drag reduction speed-adaptive fairing, which includes a curved inner fairing adjacent an upper rectangular corner at the forward end of a vehicle, together with a spaced air foil member providing high lift characteristics and a convergent path between the members. The combination is said to reduce air resistance and flow separation from top and sides of a vehicle, thus reducing air drag. It is believed that Elder's speed-adaptive fairing is most effective at an optimum speed, whereas at higher or lower speeds the speed-adaptive fairing may actually add to drag.
Another paradigm is to direct air away from high-drag portions of a vehicle. For example, a typical vehicle underbody presents a complex surface with many irregular features, which tend to trap flow and create turbulence between the vehicle and the ground. Accordingly, an air dam is conventionally used to direct oncoming air to either side of the vehicle underbody. For example, GM's U.S. Pat. No. 8,186,746 shows a passively deployable air dam for a vehicle. The passively deployable air dam includes a deployable body, which is mounted by slider bearings onto a pair of slanted shanks. At low vehicle speeds (as might be expected when moving across a surface that included potentially damaging obstacles), springs hold the deployable body in an upward retracted position on the shanks. In the retracted position a lower part of the deployable body is exposed to air flow. At sufficient vehicle speed, the air flow pushes the entire deployable body down the shanks (against the spring forces), toward a downward deployed position. As the deployable body moves down the shanks, additional area is exposed to the air flow, thereby augmenting the downward force to hold the deployable body in its deployed position. Thus, the deployable body is not positionable to any intermediate position between the retracted position or the deployed position.